A welcome addition to the city’s 70-acre public Parque La Mexicana, the green roof is adjoined by a soccer field, two basketball/volleyball courts, a padel court, and a children’s skate park connected with walking, running, and biking paths. In addition, the green roof hosts 21 varieties of native plants and grasses as well as 15 insect hotels. Through a public-private partnership, Costco constructed sprawling roofs on top of its warehouse, parking garage and loading dock for public recreational use. Three different types of roofs and surfaces were engineered, allowing a siphonic drainage and gravity drainage system that collects and transports rooftop stormwater runoff to the park’s water features. THREE BUILDING STRUCTURES The 524,549 square feet Costco was designed as three interconnected buildings. The retail warehouse is a metal building, the parking garage was created from post-tensioned concrete and the loading dock combines conventional concrete and metal decking. For the metal building portion, supplied by Butler Manufacturing, Span Construction and Engineering erected the building shell, consisting of columns, beams, roof joists, roofing panels, integral gutter system, wall girts, and metal wall panels. The structural engineer, Engineers Northwest, Inc., worked with the metal building manufacturer to reduce the number of joists and columns required, thereby creating a more seamless shopping experience with fewer structural columns breaking up the floor plate. The selection of steel, with its decreased thermal mass, as compared to a typical masonry block wall, helps reduce the building’s heat absorption, which is significant in Mexico’s hot climate. Additionally, the structure is made from 80 percent recycled steel. STRUCTURAL IMPACTS Because the green roof is not open to the public, the roof designated for the metal building warehouse didn't have to account for additional load. The base roof live load is 20 psf, the super imposed roof load is 30 psf overall and the 2-foot perimeter around the roof is 49 psf. For the parking garage, restricted to three stories to blend in with the park and meet local height requirements, post-tensioned concrete was deemed to be the most suitable material. “This is a very efficient design that utilizes less concrete than other common concrete structures while still holding heavy loads from the sports park,” reports Maribel Barba, AIA, project architect and manager, MG2 Design, Seattle. “The thickness of the floors is a little over 5 inches, which is quite thin compared to other concrete structures.”
“The rest of the receiving area was designed with steel columns and conventional concrete metal decking to hold up the weight of the children’s skate park,” she adds. For the loading dock, the project team designed a column-free span large enough to allow a semi truck to enter the space, unload its contents, make a U-turn, and exit. To enable this, MG2 custom fabricated a 90-foot-long, 12-foot-tall beam to provide the required clearance and maneuverability inside the dock area. SEISMIC REQUIREMENTS Considered to be one of the highest seismic zones in the world, the codes and standards in Mexico City and its surrounding suburbs are naturally stringent. With metal buildings, engineers must take into account providing sufficient ductility in the structure to absorb and dissipate the massive energy produced by an earthquake. Research by the Metal Building Manufacturers Association (MBMA) and industry partner American Iron and Steel Institute (AISI) evaluated the behavior of full-scale metal building frames at the University of California, San Diego (UCSD) on their shake table to study the behavior and ductility of a metal building system under various earthquake conditions. Their work revealed how metal building moment frames behave quite differently regarding ductile design philosophy and that metal building frames demonstrate little conventional ductility. Mexico City has adopted the International Building Code (IBC), which permits the use of three steel moment frame systems for resisting seismic lateral loads. Each has a different design procedure that recognizes the anticipated amount of ductility, primarily based on the inelastic rotation that can be expected at the beam-column connections.
Credit: Courtesy of Sika Technology Green roof solution with Sika Sarnafil® for the massive Mexico City project
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